1. Field of the Invention
This invention relates to a method of neutralizing an accumulated charge on a specimen when such a specimen is used as an object of observation or analysis in a scanning electron microscope or a scanning ion microprobe mass analyzer.
2. Description of the Prior Art
A scanning electron microscope is generally composed of an electron gun, a group of a plurality of lenses, a secondary electron detector, a specimen mount, an amplifier, a contrast adjusting circuit, and a cathode ray tube. Primary electrons emitted from the electron gun is converged by the lens group to be focused on the specimen as a minute beam spot. This focused minute beam spot is scanned over the specimen in synchronism with scanning of an electron beam in the cathode ray tube. On the other hand, secondary electrons generated from the specimen irradiated by the primary electrons are detected by the secondary electron detector to be used as image signals which are applied to and displayed on the cathode ray tube.
In the manner described above, the surface shape, atomic numbers and potential contrast of the specimen can be displayed on the cathode ray tube.
A scanning ion microprobe mass analyzer is generally composed of an ion source, a focusing lens system, a mass spectrometer, a specimen mount, a cathode ray tube and a data processor. A beam of primary ions emitted from the ion source is focused by the lens system to irradiate the surface of the specimen. Secondary ions generated due to emission of specimen atoms from the surface of the specimen irradiated by the primary ion beam are classified in the mass spectrometer according to their mass-to-charge ratio. Thus, identification and quantitative analysis of elements or molecules contained in the specimen can be achieved. A two-dimensional element image of the surface of the specimen can be displayed by scanning the primary ion beam over the specimen in synchronism with the electron beam scanning in the cathode ray tube, separating the secondary ions emitted from various points of the specimen in the mass spectrometer, and using signals representing specific ionic strength as image signals.
When the specimen to be observed or analyzed by the apparatus or methods described above is an insulator material, it is necessary to avoid electrostatic charging of the specimen. Generally, the secondary-emission yield .eta. of secondary electrons emitted as a result of irradiation of primary electrons on the surface of the insulator material changes depending on the energy of the incident electrons, and .eta. shows a value larger than one (1) in the energy range of 500 eV to several thousand eV. Since no electrostatic charging phenomenon occurs when the value of .eta. is larger than one, it has been a common practice to avoid accumulation of the electrostatic charge by restricting the energy of the primary electron beam to a considerably low level.
Such a prior art for prevention of electrostatic charging is intended to avoid undesirable accumulation of electrostatic charge by restricting the energy of the primary electron beam to a low level and leaves no room for adaptation of a higher energy level. As is well known in the art, the resolution of the scanning electron microscope depends upon the diameter of the primary electron beam, and the diameter of the primary electron beam changes depending on the accelerating voltage, that is, the beam energy. Thus, the prior art method used for observation of the insulator specimen has had a problem that an electron beam accelerated by a low accelerating voltage, that is, an electron beam having a low energy level must be used at the sacrifice of the resolution.
In a prior art method used for the scanning ion microprobe mass analyzer or the secondary ion mass spectrometer, irradiation of primary ion beam on the surface of an insulator specimen results in accumulation of an electrostatic charge on the surface of the insulator specimen. Therefore, the prior art method has also had such a problem that the secondary ions cannot pass a mass spectrometer due to the presence of the accumulated charge, resulting in reduced reliability of measured data. With a view to solve such a problem, a prior art publication, for example, Japanese Patent No. 996233 discloses a method in which an electron beam for charging preventive purpose is directed toward an insulator specimen in a relation superposed on an irradiating range of the primary ion beam for the purpose of analysis. This prior art charging preventive method is based on an idea that an electrostatic charge accumulated on the surface of the insulator specimen as a result of irradiation by the primary ions can be neutralized by the electron beam directed in superposed relation.
In the case of the prior art method described above, however, the condition for irradiation by the primary ion beam changes necessarily as the analysis of the insulator specimen proceeds. This variation of the condition for irradiation by the primary electron beam results in a corresponding change in the quantity of the electrostatic charge accumulated on the surface of the insulator specimen, and such a change in the quantity of the accumulating charge results also in a variation of the condition for irradiation by the electron beam used for the charging preventive purpose. Therefore, it has been difficult to optimize the condition for electron beam irradiation, for each of individual insulator specimens, and it has thus been difficult to sufficiently neutralize the electrostatic charge accumulated on the surface of the insulator specimen. Because of the difficulties described above, the prior art charging preventive method has had such a problem that the analyzed data tend to fluctuate, resulting in reduced reliability of the data.